For use in mass spectrometers, various ion sources can be considered, including inter alia plasma ion sources. A partial range of the latter relates to the ICP ion sources (ICP=Inductive Coupled Plasma), and in addition the MIP ion sources (MIP=Microwave Induced Plasma). In the case of the ICP source, a plasma is usually generated in a space surrounded by a coil, by induction. Such ion sources were in the past coupled with quadrupole analysers. The latter can be built so as to be relatively small and economic. The coupling itself is non-problematic. Both parts (ion source and quadrupole) can be operated at a potential close to ground, since the accelerating voltage required for the quadrupole is at all events in the region of a few tens of volts. No particular insulating measures are required for the specimen supply to the ICP source.
Double-focusing mass analysers were in the past coupled with various ion sources. In this case, the analyser itself was grounded. To achieve an adequate acceleration of the ions, the ion source itself was set to high voltage. This is the conventional arrangement of an ion source in a mass spectrometer having at least one magnetic sector field.
In the device known from U.S. Pat. No. 5,068,534, an ICP source is coupled with a double-focussing mass analyser operating in the conventional mode of operation. The entrance region of the analyser is, together with the plasma, at high voltage. In order to avoid breakdowns and voltages which are hazardous to the user, the induction coil of the ICP source is screened off in relation to the plasma by a special insulation. Overall, however, the high voltage existing in the region of the ICP source remains problematic for handling.
The object of the present invention is to improve the coupling, known per se, between a plasma ion source and a double-focussing mass analyser, especially to limit the voltages occurring in the region of the source.